When a Target Device uses BOOT.ISO to boot, it goes through the TSB (Two Stage Boot) process.
In the first stage of the TSB process the executable code inside the BOOT.ISO is loaded and executed. This code then downloads a second piece of code from the Provisioning Services Server in the form of a file named TSBBDM.BIN.
In the case the Provisioning Services Server software is upgraded, there will be a new version of the TSBBDM.BIN file on the server.
If the version of the BOOT.ISO is different than the version of the TSBBDM.BIN file, it can cause issues such as very slow boot times.